1. Field of the Invention
The invention relates generally to fluid treatment technology. More specifically, the present invention relates to methods and apparatus for treatment of fluids such as water to reduce the content of live microorganisms using plasma technology.
2. Description of the Related Technology
The rising concern associated with the availability of potable water is an issue that has paralleled the continual increase in global population and international development. From a global perspective, an estimated 1.1 billion people are unable to acquire safe drinking water. One cause of contamination is the presence of untreated bacteria and viruses (collectively termed microorganisms) within the water. As estimated by the Environmental Protection Agency (EPA), nearly thirty-five percent of all deaths in developing countries are related directly to contaminated water. In addition, growing populations have implemented individual water collection, water storage and water distribution units to support their water needs, including rooftop tanks and surface water collection systems. The non-circulatory nature of these units is conducive to stagnation and increased bacterial growth, and contributes to unsafe water consumption. Well water systems are also employed to obtain safe drinking water, though the water tables that support these systems are not impervious to contamination.
Currently, there are many available methods of water treatment and decontamination, including chlorination, ozonation, UV lamps, in-line filters, and pulsed electric fields. With regard to water disinfection, chlorine remains both an accepted and widely employed method of treatment. Chlorine is used to treat drinking water supplies due to its ease of use and associated efficiency with regard to inactivating microorganisms. Regardless of system size, it is one of the least expensive disinfection methods; however, its toxicity requires strict adherence to accepted concentration levels. An excess of chlorine in a drinking water supply could render the water toxic to humans. Unwanted byproducts resulting from the interaction of chlorine with other chemicals present in the water can prove corrosive and deteriorative to the system. In addition, because a chlorination based system must be continually replenished, the storage and transportation of chlorine becomes a significant hazard.
Ozonation is a growing method of water treatment; it is a process that involves ozone generation by plasma in air. The resultant ozone is bubbled into a contaminated solution through the primary mechanism of mass transport. The ozone is chemically active and is capable of efficiently inactivating microorganisms at a level comparable to chlorine. Achieving a four-log reduction in microorganisms at 20 degrees Celsius with an ozone concentration of 0.16 milligrams per Liter (mg/L) requires an exposure time of 0.1 minutes. At higher temperatures and pH levels, ozone tends to rapidly decay and requires more exposure time. Due to the corrosive and toxic nature of ozone, ozonation systems require a high level of maintenance. The efficiency is compromised by the energy that is lost during mass transport, and because ozonation requires a significant initial capital investment.
An alternative point-of-use water treatment method employs UV radiation through UV lamps. The dosage required for successful deactivation of a microorganism depends upon its structure. For example, E. Coli requires a dosage of 3,000 microwatt-seconds per cubic centimeter (μW·s/cm3) for a 90 percent reduction. The life of a UV lamp is relatively short and thus the lamp requires constant replacement. Also, the effectiveness of this treatment method is compromised by several additional factors, including biological shielding and chemical or biological film buildup on the lamp. An advantage of this system is that the temperature and pH of the treated water are not significantly affected and no undesirable by-products are created.
In-line filters are commonly used to remove undesirable substances from water. Many different types are commercially available, including carbon filters, micro-filters, and reverse osmosis filters. The key advantage to these filters is that they require no power to operate, but there are two significant drawbacks to this method. First, though these filters are capable of preventing microorganisms from passing through the system, they are incapable of inactivating them leading to bacterial growth in the filters. Secondly, the small pores needed to trap microorganisms also inhibit the flow, resulting in pressure loss across the filter. Significant pressure losses in the system require the use of larger pumps in the system.
Pulsed electric field technology is also employed to treat water. Since the electric field associated with this technology is not strong enough to initiate breakdown, there is no resulting discharge. The mechanism of electroporation caused by the electric fields effectively deactivates microorganisms. In electroporation, the electric field creates holes in the membrane of the cell, causing an influx of water and cell explosion. At nominal conditions, the energy expense for a two-log reduction of microorganisms is approximately 30 Joules per cubic centimeter (J/cm3).
The commercialization of these water treatment methods are not, however, without deficiencies. With regard to human consumption, chemical treatments such as chlorination can render potable water toxic. The effectiveness of ultraviolet radiation and ozone injection largely depends upon adherence to regimented maintenance schedules. Therefore improvements in water treatment methods, especially point-of-use water treatment systems, are needed.